Accurately georeferenced mosaics of orthophotos, referred to as orthomosaics, are becoming popular alternatives to traditional pictorial maps because they can be created automatically from aerial photos, and because they show actual useful detail on the ground.
The creation of accurate orthomosaics from aerial photos is well described in the literature. See, for example, Elements of Photogrammetry with Application in GIS, Fourth Edition (Wolf et al.) (McGraw-Hill 2014), and the Manual of Photogrammetry, Sixth Edition (American Society for Photogrammetry and Remote Sensing (ASPRS) 2013).
The creation of an orthomosaic requires the systematic capture of overlapping aerial photos of the area of interest, both to ensure complete coverage of the area of interest, and to ensure that there is sufficient redundancy in the imagery to allow accurate bundle adjustment, orthorectification and alignment of the photos.
Bundle adjustment is the process by which redundant estimates of ground points and camera poses are refined. Modern bundle adjustment is described in detail in “Bundle Adjustment—A Modern Synthesis” (Triggs et al.) in Vision Algorithms: Theory and Practice (Lecture Notes in Computer Science, Volume 1883, Springer 2000).
Bundle adjustment may operate on the positions of manually-identified ground points, or, increasingly, on the positions of automatically-identified ground features which are automatically matched between overlapping photos.
Overlapping aerial photos are typically captured by navigating a survey aircraft in a serpentine pattern over the area of interest. The survey aircraft carries an aerial camera system, and the serpentine flight pattern ensures that the photos captured by the camera system overlap both along flight lines within the flight pattern and between adjacent flight lines.
Sufficient redundancy for accurate bundle adjustment typically dictates the choice a longitudinal (forward) overlap of at least 60%, i.e. between successive photos along a flight line, and a lateral (side) overlap of at least 40%, i.e. between photos on adjacent flight lines. This is often referred to as 60/40 overlap.
The chosen overlap determines both the required flying time and the number of photos captured (and subsequently processed). High overlap is therefore expensive, both in terms of flying time and processing time, and practical choices of overlap represent a compromise between cost and orthomosaic accuracy.
The use of a multi-resolution camera system provides a powerful way to reduce overlap without excessively compromising accuracy. The capture and processing of multi-resolution aerial photos is described in U.S. Pat. Nos. 8,497,905 and 8,675,068 (Nixon), the contents of which are herein incorporated by reference. Multi-resolution sets of photos allow orthomosaic accuracy to be derived from the overlap between lower-resolution overview photos, while orthomosaic detail is derived from higher-resolution detail photos.
U.S. Pat. Nos. 8,497,905 and 8,675,068 (Nixon) describe an external camera pod attachable to a small aircraft. An external pod has two key disadvantages: the pod is highly aircraft-specific, and space within the pod is constrained. An aircraft-specific pod limits the choice of aircraft and therefore limits operational parameters such as altitude range, and, conversely, requires significant design, testing and certification effort to adapt to different aircraft. Constrained space within the pod limits the size and therefore the focal length of camera lenses, which in turn limits the range of operating altitudes for a particular target image resolution.